The invention relates to a desulfurization process and a catalytic isomerization process for hydrocarbons, particularly paraffin-containing hydrocarbon feedstreams.
Natural or straight run gasoline, i.e., naphthas, contain chiefly normal paraffins (n-paraffins) which have relatively low octane numbers. In the catalytic isomerization of paraffins, such as n-hexane and n-pentane, to equilibrium mixtures of branched chain isomers, the octane rating of the paraffins is substantially increased. Previous processes directed to paraffin isomerization have employed acidpromoted aluminum chloride catalysts, Y-type crystalline zeolite catalysts containing an elemental metal of Group VIII of the Periodic Table (U.S. Pat. Nos. 3,236,761 and 3,236,762), synthetic mordenite catalysts containing highly-dispersed platinum or palladium in the presence of hydrogen (U.S. Pat. Nos. 3,527,835 and 3,299,153), stabilized Y-sieve hydrogen zeolite compositions (U.S. Pat. No. 3,354,077), and the like. Commonly used commercial isomerization catalysts contain significant proportions of relatively expensive platinum or palladium metal components.
The hydrocarbon feedstream to an isomerization reactor is generally treated (hydrotreated) to remove sulfur and nitrogen compounds that rapidly deactivate (even poison) the relatively expensive isomerization catalyst. A common method of treating the feedstream to reduce the sulfur content is by hydrodesulfurization wherein the feed is contacted with a sulfur-tolerant hydrogenation catalyst in the presence of hydrogen. Although good sulfur removal may be achieved with hydrodesulfurization units operating under severe conditions, the efficiency of such units is ultimately limited by equilibrium and/or kinetic considerations and further desulfurization is often necessary in downstream processes.
Downstream processes employing relatively inexpensive, catalytically active materials further reduce the sulfur content of a hydrocarbon by "absorbing" sulfur therefrom under either hydrogenative or nonhydrogenative conditions. Usually, the absorbent material contains a metal component, such as nickel, copper, or silver, and the feedstocks generally treated are reformer feedstocks, particularly naphthas. Typical of such processes are that disclosed in U.S. Pat. No. 2,755,226 to Annable wherein a bed of copper molybdate pellets is used to reduce the sulfur content of naphthas. Similarly, in U.S. Pat. No. 4,224,191 to Bishop III, U.S. Pat. No. 4,204,947 to Jacobson et al. and U.S. Patent 4,582,819 to Miller, the use of copper components supported on conventional carriers is disclosed for reducing the sulfur content of reformer feedstreams. Also, processes disclosed in U.S. Pat. No. 4,419,224 to Miller include reducing the sulfur content of reformer feedstreams with promoted nickel components on alumina carriers.
Reformer feedstreams typically contain substantial proportions of hydrocarbons boiling at a temperature higher than that of n-hexane. When the sulfur content of feedstreams containing substantial portions of lighter hydrocarbons, such as n-hexane and n-pentane, is too large, appreciable loss in a catalyst's n-pentane and n-hexane isomerization activity is observed. In the present invention, it has been discovered that the isomerization activity of these relatively expensive catalysts may be improved substantially by reducing the sulfur content in the isomerization feed to below 5 ppmw, more particularly below 2 ppmw, and most particularly below 1 ppmw. Such improved activity allows a petroleum refiner to operate at milder conditions and expend less energy to produce the desired isomerized products.
Accordingly, it is an object of the invention to reduce the concentration of sulfur in a hydrocarbon feedstream to be contacted with an isomerization catalyst, particularly a catalyst useful for isomerizing feedstreams containing n-hexane and/or n-pentane, to levels at which the activity of the catalyst is significantly improved.
Furthermore, in addition to protecting the isomerization catalyst from sulfur poisoning, the petroleum refiner may also desire an increase in the capacity of an isomerization unit. Therefore, another objective of the present invention is to simultaneously reduce the sulfur concentration and isomerize n-paraffins.